diff --git a/Lib/test/test_urlparse.py b/Lib/test/test_urlparse.py

index 77c2007..c6c54b0 100644

--- a/Lib/test/test_urlparse.py

+++ b/Lib/test/test_urlparse.py

@@ -885,6 +885,12 @@ class UrlParseTestCase(unittest.TestCase):

         self.assertIn('\u2100', denorm_chars)

         self.assertIn('\uFF03', denorm_chars)

+        # bpo-36742: Verify port separators are ignored when they

+        # existed prior to decomposition

+        urllib.parse.urlsplit('http://\u30d5\u309a:80')

+        with self.assertRaises(ValueError):

+            urllib.parse.urlsplit('http://\u30d5\u309a\ufe1380')

+

         for scheme in ["http", "https", "ftp"]:

             for c in denorm_chars:

                 url = "{}://netloc{}false.netloc/path".format(scheme, c)

diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py

index 243f470..da3a40c 100644

--- a/Lib/urllib/parse.py

+++ b/Lib/urllib/parse.py

@@ -322,13 +322,16 @@ def _checknetloc(netloc):

     # looking for characters like \u2100 that expand to 'a/c'

     # IDNA uses NFKC equivalence, so normalize for this check

     import unicodedata

-    netloc2 = unicodedata.normalize('NFKC', netloc)

-    if netloc == netloc2:

+    n = netloc.rpartition('@')[2] # ignore anything to the left of '@'

+    n = n.replace(':', '')        # ignore characters already included

+    n = n.replace('#', '')        # but not the surrounding text

+    n = n.replace('?', '')

+    netloc2 = unicodedata.normalize('NFKC', n)

+    if n == netloc2:

         return

-    _, _, netloc = netloc.rpartition('@') # anything to the left of '@' is okay

     for c in '/?#@:':

         if c in netloc2:

-            raise ValueError("netloc '" + netloc2 + "' contains invalid " +

+            raise ValueError("netloc '" + netloc + "' contains invalid " +

                              "characters under NFKC normalization")

 def urlsplit(url, scheme='', allow_fragments=True):